Convert from char to int OR Convert from int to char

//This is a feasible answer for  '1_SRM_144DIV1-300' on topCoder.
class BinaryCode{
public:
vector<string> decode(string message)
{
int len = message.size();
vector<string> vec;
if(len == 0){
cout << "No data" << endl;
return vec;
}
string zeroBg = convert(message,'0'); //assume the original binary code begin with '0'
string oneBg = convert(message,'1');  //assume the original binary code begin with '1'
vec.push_back(zeroBg);
vec.push_back(oneBg);
return vec;

}

private:
string convert(const string& message,char guess){
int len = message.size();
string d_str(len,'0');//allocate len size space for d_str ,which is used to store original binary code
d_str[0] = guess;
for(string::size_type i = 1; i < len; ++i){
if(i == 1)
//将char类型转换为int类型，调用自定义的函数cToi,从整形转换为字符类型可使用在char类型的'0'的基础上加一整形实现。
//例如：将int型的2转换为字符型的'2',可采用'0' + 2,或者将char型的'2'转换为int型的2，可采用'2' - '0';
d_str[i] += cToi(message[i - 1]) - cToi(d_str[i - 1]);
else
d_str[i] += cToi(message[i - 1]) - cToi(d_str[i - 1])- cToi(d_str[i - 2]);
if(cToi(d_str[i]) >= 2){
d_str = "NONE";
break;
}
}
if(d_str != "NONE")
if( (len == 1 && message[0] != d_str[0]) || (cToi(message[len - 1]) != cToi(d_str[len - 1]) + cToi(d_str[len - 2])) )
d_str = "NONE";
return d_str;

}
size_t cToi(char ch){
return ch - '0';
}

}